PRODUCTION OF BACTERIAL RACES 115 



or if, on the other hand, a potato culture be exposed to a tempera- 

 ture of 55 to 57 C. for a few minutes, in either case fresh cultures 

 made in series upon potato at the ordinary temperature may for 

 months remain absolutely colourless. 



It is scarcely necessary to say that the classical example of 

 this order of phenomenon is to be found in the remarkable observa- 

 tions of Pasteur, Chamberland, and Roux upon the attenuation 

 of the anthrax bacillus. 1 The anthrax bacillus may be kept 

 (in broth) at a temperature of 42-5 to 43 C. for a week or more 

 without there being any noticeable diminution of the virulence 

 of the growth. But after this the virulence slowly and steadily 

 diminishes. Made under ordinary conditions from a growth 

 treated thus for twelve days cultures will no longer, when in- 

 oculated, cause the fatal disease in sheep. After thirty-one days 

 the cultures are so feeble that the very susceptible guinea-pigs 

 survive, and mice alone are killed. In forty days or so the 

 bacilli subjected to the above-mentioned temperature are en- 

 feebled to the point of death, and no further cultures from the 

 original broth are possible. What is more, each successive 

 growth obtained daily after the eighth day from a culture sub- 

 jected to the Pasteur process, if periodically resown, under 

 ordinary conditions (in alkaline beef-broth at 35 to 37 C.), 

 retains permanently, for months and years, the grade of 

 diminished virulence that had been impressed upon the original 

 culture. 



Several other methods have been suggested for attenuating 

 the anthrax bacillus, all of them possessing the advantage of 

 being more expeditious, 2 but all, save one, labour under the 

 disadvantage that the lowering of virulence produced is unstable, 

 and upon continued cultivation the successive growths vary 

 and are liable to regain the pathogenic property of the original 

 culture. The exception is Roux's method, 3 in which, by the 

 addition of minute quantities of potassium bichromate or carbolic 

 acid to the culture medium, a race is, after a short time, developed 

 which has lost the power of spore-formation. According to the 

 duration of the action of either antiseptic, and the amount of 



1 Pasteur, Chamberland, and Roux, Comples rendus, xcii., 1881. 



2 See Toussaint, Comptes rendus, xci., 1880 ; Chauveau, Cotnptes rendus, 

 xcvi., 1883, p. 678 ; and Wossnessensky, Comptes rendus, xcviii., 1884, 

 p. 314. 



3 Roux, Annales de TInst. Pasteur, iv., 1890, p. 1. 



